Combined degassing and flotation tank

ABSTRACT

A combined degassing and flotation tank for separation of a water influent containing considerable amounts of oil and gas. A rotational flow is created in the tank which forces the lighter components such as oil and gas droplets towards an inner concentric cylindrical wall where they coalesce and rise to the surface of the liquid and are removed via the outlet. The heavier parts are forced down where the heavy particles sink to the lower part where they can be removed as a sludge. The water is discharged via an outlet in the lower part of the tank. The combined degassing and flotation tank is particular suited for use in oil production at sea for removal of oil and gases from water streams before the water is returned to the sea.

This is a divisional application of patent application U.S. Ser. No. 10/363,127 filed Jul. 23, 2003 now U.S. Pat. No. 7,144,503, the priority of which is hereby claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to separation of a mixture comprising water, a fluid not miscible with water and having a lower density than water, and a gas into these constituents.

In particular the present invention relates to a combined degassing and flotation tank, which is particularly suited for use in separation processes where a water phase containing oil and gas is separated into these constituents.

The invention is applicable within a broad range of industries comprising the petroleum industry, including oil exploration and production, refining, transport, storage, cleaning of tanks and/or equipment etc.

2. Description of the Prior Art

In the oil industry the production of crude oil involves pumping a mixture of oil, gas and water from subterranean reservoirs. At the well-head a initial separation usually takes place, and the crude oil still containing some gas and water is treated in one or more separators to remove more water and gas before the crude oil is ready for refining.

At offshore oil production the water phase coming from the separation at the well-head or subsequent separators is usually discharged into the sea after a cleansing that involves the partial removal of gas, oil, chemicals and other impurities. Today this cleansing is accomplished using large equipment such as oil/gas separators, flotation tanks, hydro cyclones and degassing tanks occupying a substantial space at the production platforms.

With aging of the oil fields, one often finds that the volume of water accompanying the oil becomes much larger and consequently the capacity for water treatment often needs to be increased to treat the increasing amounts of water.

Further, there is a general concern of the pollution caused by oil production at sea, particularly when the oil production takes place in areas that are considered to be environmentally fragile, such as arctic areas or fishing areas. In the oil industry there is a fear that a demand for a significantly lower limit for the outlet of oil would make oil production from a number of the known reservoirs to be economically unfeasible if one has to rely on the equipment used at present. Thus great efforts have been made by industry and authorities to find ways to reduce the outlet of oil during oil production at affordable prices.

On oil production platforms intended for operation at sea, very limited space is usually available. Therefore there are very strict constraints on the space available for installation of new equipment that may be needed in order to handle increasing amounts of water, in particular if one should honor the rising concerns for the environment and reduce the pollution. An even stricter constraint on space may be encountered if one considers the establishment of oil production at the sea bed level.

In the prior art a number of oil-gas-water separators are known. In U.S. Pat. No. 4,424,068 a separator and a method for separating a mixture of oil, gas and water, such as may be received from an oil well, is described. The separator is in the form of a vessel divided into separation chambers and provided with a number of baffles and a dynamic separator where the incoming mixture changes direction several times. Despite the fact that this separator has been known for several years it seems not to have been widely used. Further as the separator comprises several chambers and many parts, the maintenance will be time consuming which may lead to costly stop of oil production.

U.S. Pat. No. 4,364,833 describes a separator comprising one sector containing numerous plates arranged horizontally and parallel in order to confer coalescence of small oil drops to larger oil drops, and a second sector containing numerous plates arranged vertically and parallel in order to let the formed larger oil drops rise to the top where they form an oil layer which is collected. As the separator contains a number of narrow passages between the parallel plates, these passages are susceptible to clogging by solids contained in the incoming flow, which may lead to the need to stop for cleaning.

U.S. Pat. No. 3,797,203 describes a separator containing a number of conically formed screens on the surface of which small oil droplets coalesce to larger oil drops which rise to the top of the separator where they can be collected as an oil phase.

WO 99/20873 describes a sand trap that may be placed on an oil well in order to remove heavier particles such as sand before further processing of the crude oil. The device has a mouth towards a relatively narrow part of a tank with a spatial connection towards a relatively widened part of the tank where sand and heavy particles precipitate.

Despite the number of known oil-gas-water separators there is still a need for an improved oil-gas-water separator with a better separation of the phases, which does not need a large space, which can be continuously operated with low requirement for maintenance and which can be manufactured and operated at moderate price.

SUMMARY OF THE INVENTION

It has now been found that these needs may be fulfilled by using a combined degassing and flotation tank including an essentially cylindrical vertical tank 1, a tangentially arranged inlet 2, at least one outlet 3 for gas and oil placed in the upper part of the tank, an outlet 4 for water placed in the lower part of the tank and an inner concentric cylindrical wall forming a flotation and degassing zone between the inner concentric wall and the wall of the tank in the upper part of the tank.

It has turned out that the combined degassing and flotation tank according to the present invention can perform the desired separation of an oil/gas phase from a water-phase with a surprisingly high efficiency. In use for water treatment in oil production, remaining oil and gas can be removed from the outgoing water phase providing an effluent with a very low content of hydrocarbons simultaneously with the removal of sand and other particulate materials. Further, as the throughput of the tank is very high, the space requirement is very modest in relation to the amount of treated water.

In one preferred embodiment the inner concentric cylindrical wall is formed as an inner cylinder placed in the upper part of the tank leaving an open space between the cylinder and the top of the tank, and further including an inlet guide vane 11 placed between the tank 1 and the inner cylinder 10 leaving an open space between the inner cylinder 10 and the inlet guide vane 11, and a structure for establishing calm streaming around the outlet 4 for water.

In another preferred embodiment, the combined degassing and flotation tank according to the present invention includes an essentially cylindrical vertical tank 1, a tangentially arranged inlet 2, an outlet 3 for gas and oil placed in the upper part of the tank, an outlet 4 for water placed essentially in the centre of the bottom, an essentially concentrically arranged vertical first baffle 5 extending from the top of the tank leaving a passage for water in the lower part of the tank and a passage for gas in the upper part of the tank and an essentially concentric, cylindrical vertical inner baffle 6 of a smaller diameter than the first baffle 5 and extending from the bottom of the tank leaving a passage in the top portion of the tank.

In another preferred embodiment a filtration unit, such as a unit of the adsorption filter type, is connected to the outlet of the combined degassing and flotation tank, which embodiment leads to a very efficient and high removal of hydrocarbons from the water.

The combined degassing and flotation tank according to the invention is particularly suited for treating the water phase originating from the well-head or from subsequent separators before the water phase is discharged to the sea at offshore oil production plants.

Thus, the use of the combined degassing and flotation tank in oil production forms another preferred aspect of the present invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. is a schematic section through a combined degassing and flotation tank according to the present invention.

FIG. 2. is a schematic section through a combined degassing and flotation tank according to the present invention provided with a centrally placed screen 7 and an outlet for sludge 8, showing the flow in the tank.

FIG. 3. is a schematic section through a combined degassing and flotation tank according to the present invention where an inner cylinder 10 is placed essentially in the upper part of the tank, leaving an open space between the cylinder and the top of the tank, and extending approximately ⅔ down into the tank. The tank is further provided with an inlet guide vane 11 and a horizontal circular plate 12 is placed in the lower part of the tank, leaving an open space between the plate and the wall of the tank.

FIG. 4. is a graph associated with the experiment indicated in the first example showing the efficiency of the combined degassing and flotation tank according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

The invention is based on the recognition that flotation of oil drops is significantly facilitated by the simultaneously rising of gas bubbles. Further, as the incoming fluid in the tank according to the present invention is conducted into an upwards spiralling movement, the rotation causes the lighter constituents such as oil drops and gas bubbles to be forced towards the centre of the tank until the inner concentric cylindrical wall is met where oil bubbles and gas bubbles will coalesce and rise due to their lower density than the surrounding water.

Simultaneously sand and other heavy particles that may be entrained by the inlet mixture are forced towards the wall of the tank 1 and fall to the bottom.

Thus according to the invention a flotation and degassing zone is formed in the space between the tank and the inner concentric cylindrical wall in the area from the inlet and upward.

The inlet for water is arranged essentially tangentially in order to provide a rotational movement of the water in the tank.

An inlet guide vane 11 may be provided in connection to the inlet 2 for contaminated water. In this connection, an inlet guide vane is a vane arranged on the wall of the tank in order to guide the incoming water into an upward spiralling flow. The inlet guide vane may be short or long and extending from 30-330° of the circumference of the tank, preferred 90-300°, more preferred 180-300°, and most preferred around 270°. The inlet guide vane is placed so that an open space is found between the vane 11 and the inner concentric cylindrical wall.

The incoming water is let along the vane until the end of the vane where the water continues in a fan-like pattern, with a part of the water being “recirculated” in another round along the vane while the remainder of the water escapes the circulation. The recirculating fraction is to a large extent determined by the inclination of the vane. By experiments it has been determined that a suitable inclination for the vane is in the range of 5-15°, more preferred 5-10° and most preferred approximately 7°. At this inclination, a suitable recirculated fraction is obtained without unduly obstructing the flow.

The term “in connection to the inlet” is intended to mean that the inlet guide vane is arranged so the vane meets the incoming water and guides the incoming water into an upwards spiralling flow.

The outlet for water is placed in the lower part of the tank. It is preferred to place the outlet in a location where the water is streaming calmly in order to ensure that no oil or gas droplets are entrained because of high streaming velocity. A structure for establishing calm streaming around the outlet may be provided in order avoid entrainment of oil drops by the rapid water stream. This structure may be designed as a horizontal circular plate leaving an open space between the plate and the tank if the outlet is placed essentially in the centre of the bottom, or it may be designed as a downward open ring providing shelter for the outlet if the outlet is placed as one or more outlets in the lower part of the side of the tank.

Depending on the location of the outlet the solid material falling into the bottom of the tank may form a sludge. The deposition of sludge and solid material in the lower part of the tank is disadvantageous as the separating capacity of the tank may be lowered.

If the outlet for water 4 is placed at the lowest point of the tank, the falling solid material will mainly be entrained by the water streaming out and thus removed, provided that the tank is not provided with any baffles extending from the bottom. If the outlet for water is placed in a location that allows deposition of sludge, it is preferred to provide the tank with an outlet 8 for sludge. The outlet for sludge may be designed for continuous or discontinuous removal of sludge.

One or more outlets for oil and gas may be provided. If more than one outlet for oil and gas is provided one of these outlets may serve as an outlet for gas whereas another may serve as an outlet for oil.

In connection with the present invention the term “essentially-cylindrical” means that the tank is substantially circular and the top and bottom of the tank is plane or curved. In use the tank is placed so that the axis of the cylinder is essentially vertical.

In one embodiment of the invention as shown in FIG. 3, the combined flotation and degassing tank is provided with an inner cylinder 10 placed in the upper part of the tank leaving an open space between the cylinder and the top of the tank, and further comprising an inlet guide vane 11 placed between the tank 1 and the inner cylinder 10 leaving an open space between the inlet guide vane and the inner cylinder and a horizontal circular plate 12 leaving a passage for water between the plate and the tank.

The inner cylinder 10 is arranged so that passage of oil, gas and water is allowed over the top of the cylinder. The cylinder may extend approximately ½-¾ down into the tank, preferably approximately ⅔ down the tank.

The ratio of the diameter of the inner cylinder to the tank may be selected between wide limits so that the passage of water is not limited. Preferably this ratio is selected between 0.3 to 0.75, more preferred 0.4-0.6 and most preferred approximately 0.5.

In another embodiment as shown in FIGS. 1 and 2, a tank 1 is provided with a inlet 2 placed in the lower part of the tank. The inlet 2 is arranged tangentially, which causes the incoming water to rotate in the tank as illustrated in FIG. 2. This rotation creates a centrifugal force which forces the lighter oil drops towards a baffle 5, where they coalesce forming larger drops that eventually rise and are gathered on the top of the liquid between the tank wall and the first baffle 5. Here the oil is removed via an outlet 3 for oil and gas.

The water is forced down under the first baffle 5 and up between the first baffle and the outlet 4 and/or the inner baffle 6. The water rises to a certain level, and leaves the tank via the outlet 4.

The baffles are formed essentially as cylinders open in one end and with the other end closed by the top or the bottom of the tank.

The first baffle 5 extends from the top of the tank and may be placed forming a gap between the first baffle 5 and the top of the tank. The first baffle 5 may be formed having a uniform diameter along the total height of the baffle, or it may be formed having a larger diameter in the lower end in order to provide a maximal rotational speed of the liquid in the inlet zone.

The inner baffle 6 extends from the bottom of the tank and is in one embodiment formed by an extension of the water outlet 4. In the combined degassing and flotation tank according to the invention, the inner baffle 6 provides a function as an overflow for the tank determining the water level of the tank. The inner baffle 6 may be formed by the outlet pipe 4 extended to the desired water level.

It is preferred that the water level is predetermined to the same level as the outlet for oil and gas 3 in order to obtain a continuous removal of oil and gas from the tank avoiding accumulations which may lead to reduced capacity for separation.

Between the first baffle 5 and the inner baffle 6 may be inserted further baffles, which preferably are arranged extending alternatively from the top and the bottom of the tank. Such baffles will force the water to make extra travels up and down the tank, which may allow more gas to escape from the water phase.

Each baffle is arranged substantially concentrically with the tank, and should be designed to allow passage of gas in the upper end.

The tank is operated at low pressure in order to let the gas escape the water phase. By low pressure is meant a pressure below 10 bar, such as less than 5 bar or even at atmospheric pressure. By such a low pressure most of the gas will form bubbles in the zone around the inlet (inlet zone) and will as the oil drops be forced to the first baffle 5 where they rise to the top of the tank where the gas leaves via the outlet for oil and gas.

The formation and rise of gas bubbles in the inlet zone further mimics the effects created in traditional flotation tanks where the rise of bubbles of added air leads to an enhancement of the separation of oil from the mixture. It is believed that bubble formation and the subsequent rise of the formed bubbles in the inlet zone of the combined degassing and flotation tank according to the invention contributes to the surprisingly high separation efficiency observed.

Due to the beneficial effect of the rising gas bubbles it can be useful to inject additional gas in the contaminated water before entering into the combined degassing and flotation tank, if the amount of gas in the water is low. It is within the skills of the person skilled in the art to determine the optimal amount of gas in a given contaminated water source and to determine if injection of gas may be advantageous in order to obtain a desired cleansing.

The combined degassing and flotation tank may be provided with a screen to remove fine particles such as asphaltenes, from the effluent. The screen may be placed integrally in the tank as a central cylindrical screen 7 forming a tube around the outlet 4, it may be placed directly mounted to the outlet or as a separate unit outside the tank connected to the outlet.

In one embodiment the inner baffle 6 is formed by the screen 7, in which case the water level is determined by the flow rate through the screen 7.

The screen is made as conventional screens using known materials and designs for screens,. as would be known to the person skilled in the art.

In the case of a combined degassing and flotation tank equipped with a inner baffle 6 and a central cylindrical screen 7, the tank is conveniently provided with a outlet for removal of material retained by the screen (not shown in the figures) placed in the bottom next to the outlet 4 for the effluent.

The person skilled in the art will appreciate that the combined degassing and flotation tank according to the invention may be used for 2, 3 or 4 phase separations where the phases may be selected from: water, oil, gas and solid materials, where at least one of the phases is liquid under the condition of operation.

The dimensions of the combined degassing and flotation tank may be selected depending on the amounts of water intended to be treated. In operation it has been found that the residence time in the tank for a liquid to be treated may be selected from between 5 and 300 seconds, preferably 5-150 seconds, more preferred 10-60 seconds, even more preferred 10-40 seconds. A particular preferred residence time is about 20 seconds.

For the combined degassing and flotation tank according to the invention, an efficient flotation volume may be calculated as the volume of the space bounded by the tank 1 and the height of the liquid in the tank. Based on the residence time the capacity of the tank may be calculated e.g. a tank with a efficient flotation volume of 1 m³ and a residence time for the liquid of 20 seconds has a capacity of 180 m³ per hour.

The ratio of height to diameter of the tank can be selected within wide limits preferably in the range of 1:1 to 4:1 and more preferred from 1:1 to 2:1.

It is within the skills of the person skilled in the art to select materials used for the construction of the tank based on the actual conditions for the intended use, such as the amounts of liquid to be treated, the composition of the liquid, the selected pressure, the temperature of the liquid and the presence of possible corrosive chemicals in any of the phases of the mixture.

In the way the combined degassing and flotation tank is formed, all surfaces are vertical or at least have a steep inclination or are subjected to a rapid flow, which prevents deposit of solids, with the exceptions of the surfaces in the sectors intended for collection of particulate material and sludge, which sectors also preferably have outlets for removal of these materials. Further, no narrow passages are present in the tank. Consequently there is no place in the combined degassing and flotation tank according to the invention, which is susceptible to clogging by solid materials. Therefore the combined degassing and flotation tank may be operated continuously without, or only with a minimal need for, maintenance. The necessary maintenance, such as replacement of a cylindrical screen if so provided, can easily be performed from the top of the tank, which preferably is constructed to be removable. Thus the combined degassing and flotation tank according to the invention has a remarkable robustness i.e. it can be run for long periods without interruptions, and the few stops that may be required for maintenance can be made short. The high capacity combined with the small occupied space and the robustness of the combined degassing and flotation tank according to the invention makes it particularly suited for use at off-shore oil plants such as oil production platforms. Further it is also well suited for use in oil production in plants located on the sea bed because, at such a location, the constraints on space may be even stricter than on traditional oil production platforms and the capacity for maintenance may be lower.

Additives such as flocculants may also be added to the water before entering into the combined degassing and flotation tank according to the invention in order to improve the cleaning efficiency.

In use the combined degassing and flotation tank according to the invention routinely reduces the oil content of an oil/water mixture to 20 mg/l or less starting from several hundred mg/l e.g. 200-800 mg/l.

Full-scale tests (more that 150 m³/h) have shown that the oil content can be lowered from several thousand ppm down to approximately 10 ppm and leave the water essentially free of gas.

In a preferred embodiment the combined degassing and flotation tank is used in combination with a filtration unit, preferably of the adsorption/absorption type, which filtration unit may further reduce the oil content of the effluent water. A particularly suited filtration unit for such a combination is the filtration unit disclosed in the European Patent application No. 00610080.4. Starting from an oil/gas/water mixture containing 400-800 mg hydrocarbons per litre, a reduction of hydrocarbon content to less than 20 mg/l after the combined degassing and flotation tank and a further reduction to 5 mg/l or less after the filtration unit can routinely be obtained, while high throughput is maintained.

Even though the combined degassing and flotation tank has been described mainly with respect to uses in oil production, the invention is not limited to such uses but can be used within a broad range of industries where separations of a liquid mixture of water, a liquid insoluble in water and a gas takes place.

Now the invention is described by an example, which should not be regarded as limiting for the invention.

EXAMPLES Example 1

A combined degassing and flotation tank corresponding to FIG. 2, having a diameter of 500 mm and a height of 1200 mm and an efficient flotation volume of 125 litre, was tested on water phase effluent from a second step oil separator from a commercial oil production platform. The sample water contained varying amounts of oil and gas in the range equivalent to approximately 50-200 mg hydrocarbon per litre. The intake varied between 1.8 and 9.5 m³/h.

The output water contained approximately 20 mg hydrocarbon per litre or less and, during most of the experiments, less than 20 mg/l. The efficiency of cleaning calculated as percentage hydrocarbon removed was between 80 and 90% during most of the experiment.

The actual data is shown in FIG. 4, which is a graph showing the concentrations of hydrocarbons in the inlet and the effluent stream of the combined degassing and flotation tank measured at regular intervals during the experiment.

Example 2

A combined degassing and flotation tank was designed essentially as depicted in FIG. 3 having the following dimensions, with the inner cylinder extending approximately ⅔ down into the tank:

Height 2530 mm Diameter of tank 1130 mm Diameter of inner cylinder appx. 500 mm

This combined degassing and flotation tank was run in a full scale with an inlet of 150 m³/h.

The plant was operated continuously without breaks for more than 6 months with an inlet of water contaminated with approximately 200-600 ppm hydrocarbonaceous oil and gas and an outlet containing 5-10 ppm. This study confirms the high throughput with high cleansing efficiency essentially without any need for intermittent breaks for maintenance.

The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be recognized by one skilled in the art are intended to be included within the scope of the following claims. 

1. A method of separating a water mixture that includes water, hydrocarbonaceous oil not miscible with and having a lower density than water, and gas, comprising the step of separating hydrocarbonaceous oil and gas from the water using a combined degassing and flotation tank having an essentially cylindrical vertical tank, a tangentially arranged inlet, at least one outlet for gas and oil in the upper part of the tank, an outlet for water placed in the lower part of the tank, and an inner concentric cylindrical wall forming a flotation and degassing zone between said inner concentric wall and the wall of the tank in the upper part of the tank, said combined tank being operated at a pressure below 10 bar.
 2. The method according to claim 1, further comprising injecting additional gas into the water mixture entering said tank before said mixture enters the tank.
 3. The method according to claim 1, further comprising injecting a flocculant into the water mixture entering said tank before said mixture enters the tank.
 4. The method according to claim 1, further comprising using said tank in oil production at sea level or at sea bed level.
 5. The method according to claim 1, further comprising using said tank in combination with a filtration unit connected to the water outlet of said tank.
 6. The method according to claim 3, further comprising using said tank for treating water from which a portion of oil and gas has been removed previously by conventional means.
 7. A method of phase separation of a mixture of at least two phases comprising at least the steps of: a) introducing the mixture to a tank through a tangentially arranged inlet; b) separating phases of the mixture in a flotation and degassing zone formed in a space between a tank wall and an inner concentric cylindrical wall; c) outputting at least one phase through at least one outlet in a lower part of the tank; d) outputting at least one other phase through at least one outlet above the inlet to the tank; said mixture introduced into the tank being guided in a mainly upwards spiralling movement in said space between the tank wall and the inner concentric cylindrical wall, said at least one phase being selected from the group consisting of water and solid materials, and said at least one other phase being selected from the group consisting of oil and gas, at least water being liquid under conditions of operation, and said tank being operated at a pressure below 10 bar.
 8. The method according to claim 7, wherein the phases are oil, water and gas.
 9. The method according to claim 7, wherein the at least one phase is water and the at least one other phase is oil and gas.
 10. The method according to claim 7, wherein the upward spiralling movement is controlled by an inlet guide vane.
 11. The method according to claim 7, wherein the mixture has a residence time in the tank between 5 to 300 seconds.
 12. The method according to claim 7, wherein the pressure is below 5 bar.
 13. The method according to claim 7, wherein additional gas is injected into the mixture.
 14. A method of separating in a tank a mixture comprising water, hvdrocarbonaceous oil, and a gas into these constituents, wherein the method comprises the steps of: conducting incoming mixture into a tangentially arranged, mainly upwards spiralling movement between a tank wall and an inner concentric cylindrical wall; taking out gas and said oil in an upper part of the tank; taking out water and possible solids in a lower part of the tank; and said tank being operated at a pressure below 10 bar allowing the gas to escape the water phase and form gas bubbles, said gas bubbles enhancing the separation of said oil from the mixture.
 15. The method according to claim 14, wherein the upward spiralling movement is controlled by an inlet guide vane.
 16. The method according to claim 14, wherein the mixture has a residence time in the tank between 5 and 300 seconds.
 17. The method according to claim 14, wherein additional gas is injected into the mixture.
 18. The method according to claim 7, further comprising the step of using said method for purifying water.
 19. The method according to claim 7, further comprising the step of using said method for purifying water from oil.
 20. The method according to claim 7, further comprising the step of using said method for purifying water in an oil producing plant.
 21. The method according to claim 14, further comprising the step of using said method for purifying water.
 22. The method according to claim 14, further comprising the step of using said method for purifying water from oil.
 23. The method according to claim 14, further comprising the step of using said method for purifying water in an oil producing plant.
 24. The method according to claim 1, wherein said water mixture is coming from separation at a well-head at offshore oil production.
 25. The method according to claim 24, wherein said water mixture is coming from an aged oil field.
 26. The method according to claim 1, wherein said water mixture is coming from separation at separators subsequent to a well-head at offshore oil production.
 27. The method according to claim 26, wherein said water mixture is coming from an aged oil field.
 28. The method according to claim 1, wherein said water mixture is coming from an aged oil field.
 29. The method according to claim 1, wherein said water mixture is coming from an oil field in an arctic area.
 30. The method according to claim 1, wherein said water mixture is coming from an oil field in a fishing area.
 31. The method according to claim 1, wherein the mixture introduced into the tank is guided in a mainly upwards spiralling movement in said flotation and degassing zone.
 32. The method according to claim 1, wherein the incoming water is guided into an upward spiralling flow by an inlet guide vane.
 33. The method according to claim 1, wherein the oil content in the mixture is reduced to less than 20 mg/l.
 34. The method according to claim 7, wherein the oil content in the mixture is reduced to less than 20 mg/l.
 35. The method according to claim 14, wherein the oil content in the mixture is reduced to less than 20 mg/l. 